1. Field of the Invention
The present invention generally relates to a system for avoiding lightning strikes. In particular, it relates to a lightning arresting system including a lightning rod and a conductive underground network connected to the rod.
2. Description of the Related Art
As is known, lightning occurs when a region of the atmosphere acquires a potential difference great enough to overcome the resistance of the air. Specifically, when lightning occurs, an electric dipole structure is produced in a cloud in a manner such that a large net-positive charge exists in the upper region of the cloud and a large net-negative charge in the lower region, as shown in FIG. 1. (To be precise, there exists a small net-positive charge in the lowest region of the cloud.)
A cloud-to-ground flash includes at least two strokes: a leader stroke and a return stroke. A leader stroke (carrying a negative charge) passes from cloud to ground.
The leader stroke is often stepped and has many branches extending out from the main channel. As it nears the ground, it induces an opposite charge at the point AP to be struck (called xe2x80x9cantipole pointxe2x80x9d hereinafter). Then, a return stroke (carrying a positive charge) from ground to cloud is generated through the main channel.
The two strokes will meet at a certain height above the ground. At this juncture, the cloud is short-circuited to the ground and a highly luminous return stroke passes through the channel to the cloud. A typical lightning flash involves a potential difference between cloud and ground of several hundred million volts, with peak currents on the order of 20,000 amperes.
Lightning strikes are deadly dangerous so that lightning rods are often used for protecting structures from being struck. As shown in FIG. 1, a lighting rod 1, which is made of copper for example, is usually disposed at a high point on a structure ST to be protected. The lighting rod 1, connected to the ground GR by low-resistance cables 2, is expected to intercept flashes and transmit their current to the ground safely.
The conventional lightning rod 1 is simply put at the top of the structure ST since it is uncritically believed that lightning tends to strike the highest object in the vicinity. In this connection, a theory says that a lightning rod can provide a cone of protection whose ground radius approximately equals its height above the ground (that is, the protection angle is about 45 degrees.) Actually, however, structures are sometimes struck and damaged by lightning, even though they are within the protection cone. This shows that conventional lighting rods are not reliable enough to protect structures from lighting strikes.
The present invention has been proposed under the circumstances described above. The inventor of the present invention is of the opinion that the cause of the failure of the conventional lightning rods to intercept lightning resides in the uncritically simple grounding manner.
It is, therefore, an object of the present invention to provide a lightning-arresting system designed to provide more reliable protection against lightning than is conventionally possible.
According to the present invention, there is provided a lightning-arresting system including a lightning rod and an underground conductor connected to the rod, wherein the underground conductor is spread to cover a protection area.
Preferably, the underground conductor may include a conductive network.
Preferably, the network may include a plurality of rectangular meshes each having a side length which is no greater than 1 meter.
Preferably, the plurality of rectangular meshes may include meshes different in size from each other.
Preferably, the meshes different in size may include an inner mesh and an outer mesh, wherein the inner mesh is greater than the outer mesh.
The lightning-arresting system may further include a connection cable which connects the lightning rod to the underground conductor. The connection cable may be connected to the center of the underground conductor.
The lightning-arresting system may further include a conductive post which connects the lightning rod to the underground conductor. The conductive post may be one of the vertical steel supports of the structure to be protected from lightning.
In the above case, the underground conductor may be formed with an opening for avoiding interference with the conductive post.
Preferably, the underground conductor may be laid under the structure to be protected, wherein the underground conductor is larger than the building area of the structure.
The lightning-arresting system of the present invention may further include a common connection point to which the lightning rod is connected. In this case, the underground conductor may include a plurality of linear conductive members or lines each extending radially from the common connection point.
Preferably, each conductive line may include some branches.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.